In transport means such as trains and buses, there is an increasing requirement both for connections to a broadband Internet service and for small low-cost high-performance antennas.
Currently, it is known to produce a satellite link between a mobile terminal and an earth station, for example to provide an Internet connection for the passengers on a train or bus, using an antenna which is not very directional, operating in the L band. The problem is that, in the L band, there are very few available frequencies and the communication transmission rate is therefore very low. To increase the rate, it is necessary to establish links with satellites operating in the Ku (10.5 GHz to 14.5 GHz) band or the Ka (20 to 30 GHz) band and to produce directional antennas. However, with a directional antenna, it is necessary for it to be continuously pointed at the satellite irrespective of the position of the vehicle.
To cover a territory such as Europe, the transmission/reception specifications for a mobile terminal capable of providing the required transmission quality lead, in the Ku band, to antenna gains typically of around 34 to 35 dB over the area covered and the antenna must be capable of being pointed, both in transmission and in reception, within an angular range between 0° and 360° in azimuth and between 20° and 60° on average in elevation.
Such performances may be achieved using an antenna array comprising elementary radiating elements, the phase of which is controlled so as achieve precise pointing in a chosen direction. These array antennas have the advantage of being flat and therefore of small size in their height direction. However, since the angular range to be covered is very large, in order to obtain good performance and to avoid the appearance of array lobes in the radiation pattern of the antenna, it is necessary to use a beam-forming array comprising a very large number of phase controls, something which is prohibited. For example, for an antenna in the Ku band having an area of the order of 1 m2, the number of radiating elements of the antenna must be greater than 15000, this being unacceptable in terms of cost and complexity of the antenna for an application in transport means.
It is also possible to point an antenna within a wide angular range using mechanical pointing. In this type of antenna, the antenna is pointed towards the satellite by a combination of two mechanical movements. A first mechanical movement is achieved by means of a rotating platform lying in a plane XY and orienting the antenna both in elevation and in azimuth. A second movement in elevation is performed by an ancillary device, for example a flat mirror fastened to the platform. The antenna conventionally includes a parabolic reflector and a radiating source illuminating the reflector. To reduce the overall size of the reflector and to reduce the height of the antenna, its periphery is elliptical instead of circular. Typically, such an antenna currently deployed on high-speed trains has a height of the order of 45 cm. Although this height is compatible with current trains, it is too large for future double-decker high-speed trains for which the available height for fitting an antenna, between the roof of the train and the catenaries, is much too small.
Moreover, for an application in the aeronautical field, the height of the antenna has an influence on the drag caused by the aircraft and on the fuel consumption. For example, current reflector antennas fitted onto aircraft have a height of the order of 30 cm and increase the fuel consumption, equivalent to eight additional passengers.
There are architectures for reducing the height of the mechanically pointed antenna. According to a first architecture, the antenna is made up of two parallel plates, between which longitudinal current components flow, and an array having a line of continuous transverse slots that couple and radiate the energy into space. The two plates and the array of slots are mounted on two coplanar mounts mechanically rotating independently of each other, the two rotational movements being superposed and carried out in the same plane of the mounts. The orientation of the lower mount is used to adjust the pointing direction in azimuth while the orientation of the upper mount is used to vary the inclination of the slots and thus modify the pointing direction in elevation of the beam generated by the antenna. However, since this antenna initially operates in linear polarization mode, it is necessary to add an additional steerable polarization grid mounted on the upper face of the antenna in order to control the plane of polarization of the antenna, thereby increasing the implementation complexity and the height of the antenna, which is therefore not flat.
According to a second low-height flat antenna architecture, the antenna comprises several alternating substrate planes and metal planes superposed one above another. The antenna comprises a first, lower metal plane, then a first substrate plane comprising several sources, the first substrate plane having a lateral end forming a parabolic surface on which the waves transmitted by the sources are reflected. Above the first substrate plane is a second metal plane having slots for coupling the reflected wave plane, each coupling slot emerging in respective slotted waveguides placed side by side so as to be mutually parallel in a same second substrate plane. The guided waves are then transmitted in the form of a radiated beam through a plurality of radiating apertures made in a third, upper metal plate. Scanning and depointing of the beam in elevation, in a plane perpendicular to the plane of the antenna, is achieved by switching the various sources, but no pointing modification in azimuth is possible. Moreover, this very compact antenna has the drawback of requiring high-power switching means, something which has never been simple to achieve. Furthermore, the sources are switched discretely, thereby preventing the beam from being continuously pointed. Finally, this very compact antenna is powered by a single power source, thereby requiring the use of bulky power amplifiers, which considerably increase the volume of the antenna, which becomes too large for an application in transport means.
To solve the problem of discrete pointing of this flat antenna, it has been proposed to use only a single source and to place the flat antenna on a rotating platform for adjusting the pointing in azimuth, the platform having an articulated mirror on the platform, the angle of inclination of which to the plane of the platform can be varied by rotation. The plane wave transmitted by the source illuminates the mirror, which reflects this wave in a chosen pointing direction, the angle of inclination of the mirror enabling the angle of elevation of the transmitted beam to be adjusted. This antenna is very elliptical, the size of the mirror in its articulated region on the platform being much greater than the size of the mirror in its inclined region above the platform, thereby making it possible to reduce the height of the antenna to 20 or 30 cm, but this height still remains too large for an application in transport means.